The invention relates to a low-temperature current transfer structure which includes a heat exchanger and which has a cold end to which a supercooled, that is, a superconductive component, is connected and an opposite warm end to which cables or current busses are connected. At the cold end, the temperature is close to the absolute zero point while the warm end is at ambient temperature.
With such current transfer structures, high to very high electric currents are supplied to low-temperature superconductive components. The transition from a material of normal conductivity to superconductive material occurs within the current transfer structure.
The current passing through normally conductive material generates heat according to Joule's principle. This heat as well as the heat transmitted from the environment by way of heat transfer has to be removed effectively and reliably so that superconductivity can be maintained. Measures which increase the surface area and a sufficiently large flow of a coolant with good heat capacity are a prerequisite herefor.
The usual designs for low-temperature current transfer structures are such that, at the cold end, the superconductor extends somewhat into the contact area of the current transfer structure and is connected to it there. Above the contact area around the current transfer structure, that is, in the actual heat exchanger area, there are provided surface-enlarging means for the transmission of heat to a coolant flowing therepast. In the normal current transfer structures such surface-enlarging means are, for example, capillaries, wires, filaments, corrugated or rolled metal sheets or wire mesh.
U.S. Pat. No. 3,610,809 discloses a gas-cooled porous electric conductor up to 450 A with nonconstant cross-section. The proposed form for the porous electric conductor is either a spirally gourd metal screen or a stretched or perforated metal sheet exposed to a coolant flow for the removal of heat therefrom.
U.S. Pat. No. 3,654,377 discloses a current conducting arrangement with a two-way conductor used for supplying current to and from a current source in a cooled load structure.
However, with all these methods, electric current transfer from the heat exchange region to the connector at the warm end takes place in an inadequately cooled area where, at the same time, a substantial amount of heat is generated by the resistance of the electric current.
Low-temperature current transfer structures of this type are mentioned in CRYOGENICS, Apr. 1975, page 197 in an article by Yu L. Buyanov et al. (see particularly column 2, fourth paragraph).
In spite of the heat generated there, the coolant is not heated up to ambient temperature because of insufficient heat exchange surface area. This leads to condensation and the formation of ice which detrimentally affects the electric high-voltage strength at those points.
With such current transfer structures substantial efforts are required to prevent the formation of ice in the area of the warm ends thereof. Two basic measures are utilized alternatively or in combination. One is the provision of heating means by which the warm end area is sufficiently heated. The other is the provision of enclosures through which dry gas is conducted. Both measures if used alone or in combination require substantial design efforts and expenses for technical and physical reasons because of the high current flow and potentially high electrical voltages.
It is the object of the present invention to provide a low-temperature current transfer structure with heat exchangers having heat exchange areas at the warm end by which icing of the current supply structure or the coolant conduit during high current operation as well as during low current operation or without current flow is avoided.